A conventional intraocular implant-type vision stimulating unit is so arranged that incoming light from outside the eye is photoelectrically converted by light receiving elements disposed in the form of a two-dimensional array, implanted beneath the retina, and adapted to effect only photoelectric conversion, and a retinal region is stimulated by electrical signals so as to obtain vision artificially.
FIG. 5 is a diagram illustrating equivalent circuits of micro photodiodes disclosed in JP-T-11-506662. In FIG. 5, filters are respectively inserted in pin photodiodes 101 and 104 so that their p sides respond to the visible light and their n sides respond to the near infrared light. The pin photodiode 101 whose n side becomes a light receiving surface with respect to incoming light 107 and the pin photodiode 104 whose p side becomes a light receiving surface with respect to the incoming light 107 are formed as a pair.
In the conventional vision stimulating unit constructed as described above, the incoming light 107 is first subjected to photoelectric conversion by the photodiodes 101 and 104, and as electron and hole pairs are generated to form a photoelectric current. Since the photoelectric current flows out from the p side, a positive/negative bipolar current, in which a negative current is obtained from an n-side electrode 103 of the pin photodiode 101 and a positive current is obtained from a p-side electrode 106 of the pin photodiode 104, is obtained by this pair of photodiodes. Accordingly, as the pair of electrodes 103 and 106 are disposed in a retinal region, it is possible to impart a positive/negative bipolar electrical stimulus. In practice, in this conventional example, the near infrared light generated outside is caused to strike the retinal surface at an appropriate timing, and is combined with the visible light 107 inputted as image information from the outer world, thereby generating a positive/negative bipolar current so as to prevent the deterioration of the body issue. For this reason, a filter for near infrared light is fitted in the pin photodiode 101. Further, as the pairs of photodiodes thus structured are disposed in the two-dimensional form, image information from the outer world is used as a stimulus pattern on the retinal surface.
In the conventional example shown in FIG. 5, characteristics of the intraocular implant-type photodiodes under ordinary room light (illuminance Eo=1,000 luxes or thereabouts) are considered by assuming that the intensity (quantity of light) of the incoming light is P, and that a photoelectric current flowing between a p-side electrode 102 and the n-side electrode 103 of the pin photodiode 101 is Iph. In a case where the eyeball is considered as a lens, if the focal length is F=6 mm and the effective aperture is D=8 mm, the f-number becomes F=f/D=2.0. At this time, if it is assumed that reflectivity of the object is R=1.0 and that the transmittance of the eyeball is T=1.0, the illuminance E at the retinal surface becomes E=60 luxes from the following:
  E  =                    R        ⁢                                  ⁢        T                    4        ⁢                  F          2                      ⁢          E      0      Accordingly, in a case where the light receiving surface of the photodiode is set to 20×20 μm2 in the case of the aforementioned publication, the quantity P of incoming light upon the photodiode becomes P=30 pW or thereabouts since 1 lux is 0.14 μW/m2 with respect to the light with a wavelength of 550 nm. If the light-receiving sensitivity of the photodiode is assumed to be S=0.1 A/W, the photoelectric current is Iph=PS=3 pA or thereabouts. It can thus be understood that an extremely small current is generated. Since the amount of charge necessary for stimulation is the to be 100 nC/m2 or more, the necessary stimulation time becomes 0.1 sec., and is therefore not realistic. In addition, the conventional example, which requires two photodiodes to output a positive/negative bipolar current, is disadvantageous to the implant-type vision stimulating unit for which the smallest possible size is required.
In addition, with this conventional example, the pulse signal cannot be generated by the circuit implanted in the eye, and there is a drawback in that a device is required for causing the infrared light generated outside to enter the eye for the purposes of generation and modulation of the pulse signal.
In view of the above-described problems of the conventional art, an object is to provide an intraocular implant-type vision stimulating unit which is highly sensitive, compact, and low power consumptive.
Another object is to provide an intraocular implant-type vision stimulating unit which is capable of outputting a bipolar pulse signal in a simple constitution.